This invention relates to an evaporative control system for an internal combustion engine and particularly to an apparatus and method of controlling the purge rate of fuel vapors from a fuel vapor collection canister.
It is conventional to use fuel vapor recovery canisters to control the loss of evaporative hydrocarbons from vehicle fuel tanks. Generally the canisters take the form of a container filled with activated charcoal or some other absorbing agent which is effective to store the evaporated hydrocarbons until they can be drawn into the induction system of the engine to undergo combustion in the engine cylinders. In these systems, the vacuum in the intake manifold of the engine is used to draw a purge stream of air through the canister so as to purge the collected vapors from the active material of the canister during each engine operation so as to condition the canister for collection of subsequently generated vapors.
It is desirable to provide high purge flow rates through the fuel vapor storage canister so that the canister is quickly purged of the absorbed fuel vapors thereby enabling the canister to have maximum working capacity when the engine is next shut down. However, if the purge flow rate through the canister is unrestricted when the canister contains a large amount of collected fuel vapors, the resulting large quantities of fuel vapor drawn into the engine induction system from the canister during high purge flow rate conditions, such as low engine speed/load conditions where the manifold vacuum level is high, results in an excessively rich air/fuel ratio of the mixture drawn into the engine. This rich mixture may affect both emissions from the engine and engine performance. The undesirable rich air/fuel ratio of the mixture would typically result even though the fuel delivery system of the engine employs closed loop control of the air/fuel ratio of the mixture delivered to the engine. This is because closed loop control systems generally have limited control authority. In other words, closed loop air/fuel ratio control systems typically have a limit in the maximum amount of adjustment that can be made in attempting to maintain a desired air/fuel ratio. When that limit has been reached as a result of large quantities of fuel vapors drawn into the engine from the canister, the closed loop controller is incapable of thereafter controlling the air/fuel ratio to the desired value until the amount of vapors being drawn into the engine from the canister decreases.
In order to prevent the air/fuel ratio of the mixture drawn into the engine cylinders from becoming excessively rich, canister purge control systems typically limit or restrict the canister purge flow rate via an orifice in the purge flow line. Generally, the size of the purge flow orifice employed to restrict the purge flow rate is selected as a compromise between a flow assuring maximum canister working capacity and a flow that assures that the authority of the closed loop air/fuel ratio controller is not exceeded so as to avoid the undesirable effects of returning large quantities of fuel vapor to the engine induction system. The selection of an orifice for achieving the foregoing is made more difficult because of the difficulty of controlling or quantifying the amount of vapor a canister may yield under continuously changing temperature and fuel RVP extremes.